Yahoo named longtime Google executive Marissa Mayer its new CEO, replacing Ross Levinsohn, who has been interim CEO since May.

The move took by surprise the many Wall Street analysts who had assumed Yahoo's board would stick with Levinsohn, a longtime Internet-advertising-media executive with stints at News Corp. and other entertainment companies.

Mayer, on the other hand, is perceived as more tech-savvy. The 20th employee at Google, she's an engineer with degrees in symbolic systems and computer science from Stanford University and is credited as an inventor on several patents in artificial intelligence and interface design.

At Google, where she was most recently vp of local, maps and location services, she had been overseeing more than 1,000 product managers and, as part of the search giant's operating committee, she was working closely with company co-founders Larry Page and Sergey Brin.

Yahoo said her position as CEO as well as a board member begins Tuesday. The announcement, issued shortly after Wall Street's closing bell on Monday, made no mention of Levinsohn and it's not known whether he'll remain with Yahoo.

Reaction to Mayer's appointment was dramatic and mixed on Monday. A headline at TheStreet.com read: "Marissa Mayer is a bad choice for Yahoo," while one at Forbes asked if Mayer were the "new most powerful woman in tech." Seeking Alpha's headline read: "Upset alert as Marissa Mayer takes over at Yahoo." Reuters called it a "surprise hire" and Wall St. Cheat Sheet called it a "big coup."

Management at Yahoo hasn't exactly been a model of stability since Terry Semel parted ways with the pioneering Internet company five years ago. Levinsohn was named interim CEO after a resume scandal ended the short reign of Scott Thompson and he was preceded by Carol Bartz. Before her, it was Yahoo co-founder Jerry Yang serving as CEO for 18 months.

Levinsohn had been considered a top contender for the CEO post along with Jason Kilar, the top executive at Hulu. Kilar, though, said two weeks ago he had taken himself out of the running, a move several Wall Street analysts -- mistakenly, as it turns out -- presumed would open the way for Levinsohn.

Instead, it's Mayer who is tasked with leading Yahoo back to some semblance of the glory it has lost with the ascension of Google.

She has her work cut out for her. Data released last week by comScore indicates that Google's share of the U.S. Internet search market rose to 67 percent in June while Yahoo's slipped to 13 percent, putting it even below Bing's share, which rose to 16 percent.

Yahoo's stock price, meanwhile, fell fractionally Monday to $15.65, down about 40 percent since Semel left the company in June 2007. By contrast, shares of Google (a company Semel once tried to purchase for $3 billion but today is worth $187 billion) are up about 14 percent in the same timeframe.